That's interesting and I may be willing to make the jump before I get too deep into BS. The one reason why I chose BS over F4 is because BS is widely used in the corporate world, especially the federal government here in the US, so I figured it would be a good skill to have (I currently mostly do design work for the Gov). However, all the frameworks are pretty similar, so it shouldn't be too difficult to make the jump. The other big issue I had with F4 is that it doesn't work for IE7 and more than half of my users still use IE8, so that's a huge audience to leave behind. Although, I'm going to read up more about the IE8 grid support section on F4's support page.
Update: I've been looking over some of the chatter on F4's IE8 support page and it seems you have to go through some hoops to get IE8 working just right: https://gist.github.com/zurbchris/5068210
I think I'd rather make the minor media querie adjustments to BS's css rather than having to deal with F4's IE8 hacks. Furthermore, BSv3 is just around the corner.